This invention relates to electrical relays, and, more particularly, to a relay that is first open, and then closes permanently upon command.
A relay is an electrical device that responds to particular input conditions and then opens or closes electrical contacts to make or break an electrical circuit through the contacts. In a typical relay, a small input signal such as a small voltage change controls one or more pairs of contacts designed to carry large electrical currents. Relays are widely used in a variety of electrical applications.
In one application, a relay is used in the bypass circuit of a spacecraft energy storage system. In such an energy storage system, a number of storage cells are interconnected together to store energy at preselected voltage and energy levels. A bypass circuit is provided to isolate storage cells that have failed, so that the failure does not cause an open circuit in the system.
The bypass circuit around each storage cell includes a relay that is normally open as long as the storage cell is operating properly. If a high voltage malfunction is detected in the storage cell, the relay operates and closes, to complete the bypass around the failed storage cell. The failed storage cell is thereby isolated from the remainder of the energy storage system, and the failure of one cell does not cause failure of the system, only a reduction in the overall energy storage capacity of the system. Spacecraft energy storage systems can be designed with an excess initial storage capacity to account for expected failures during operation.
Various types of relays have been tried in such bypass circuits. All have been found subject to relay chatter, where the current and voltage carrying contacts can spring open slightly under some conditions and cause arcing. Even conventional "latching" relays may open unintentionally in some conditions. The failed cell is thereafter no longer bypassed, and can damage other parts of the energy storage system or cause a malfunction in the power system.
There is therefore a need for an improved relay for use in the bypass circuit of a spacecraft energy storage system. The relay should be of the type that is normally open, can be closed upon command, and thereafter will remain securely closed. Such a relay should be highly reliable, and not subject to accidental opening or relay chatter. The present invention fulfills this need, and further provides related advantages.